AN938 datasheet(30/33 Pages) STMICROELECTRONICS

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Applications

AN938

30/33

Doc ID 5410 Rev 10

4.4

Current generator

Sometimes the application specifications requires generating constant current, fixed or

adjustable, for chemical processes, lamp powering, or battery chargers for lead acids, ni-cd

and ni-me-hyd batteries.

We give one suggestion for obtaining an accurate constant current generator.

The schematic in Figure 41 shows a simple solution for constant or variable current

generator, with good accuracy of the current, using a simple op/amp supplied by the 5.1 V

available reference voltage.The threshold current is fixed by the voltage divider connected to

the n.i. input op/amp, at about 100 mV. This is also the maximum voltage dropping on the

current sense resistor Rs. Adjusting the 4.7 kΩ, the threshold can be easily changed.

The formula to set the output current is:

Equation 24

Figure 41.

Constant current generator up to 3.5 A

4.5

From positive input to negative output

Figure 42 shows how to obtain a -12 V, when only a positive supply is available.

The maximum output current is given by the formula Iout = 3.5 × (1-D) according to the

relation for the buck-boost topology.

This negative output has to show good precision, stability and regulation, and it must be

protected from output short-circuit.

With the suggested application schematic, one of the aims is to guarantee the performance

as specified above and to contribute to the simplification of the power transformer, mains or

high frequency.

5.1 R

⋅

() R

⋅

-----------------------------------

AM03523v1

E/A

OUTPUT

STAGE

140

μH

6V to 12V/2A

13V<Vin<45V

PWM

5.1

LM358

μF1μF

R1 4.7K

100nF

100

R2 100

55m

V+

STPS

745D

220

μF

470

μF

7,8

111

4,5,6

13,14,15

2,3

5.1K

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